1. Field of the Invention
The present invention generally relates to fan containment assemblies for turbomachinery, such as gas turbine engines. More particularly, this invention relates to an automated apparatus and method for removing an abradable material for a fan containment assembly.
2. Description of the Related Art
Gas turbine engines generally operate on the principle of compressing air within a compressor section of the engine, and then delivering the compressed air to the combustion section of the engine where fuel is added to the air and ignited. Afterwards, the resulting combustion mixture is delivered to the turbine section of the engine, where a portion of the energy generated by the combustion process is extracted by a turbine to drive the engine compressor. High bypass turbofan engines, widely used for high performance aircraft which operate at subsonic speeds, have a large fan placed at the front of the engine to produce greater thrust and reduce specific fuel consumption. The fan compresses the incoming air, a portion of which is then delivered to the combustion chamber, though a larger portion is bypassed to the rear of the engine to generate additional engine thrust.
The fan is circumscribed by a fan containment case such that the case is immediately adjacent the tips of the fan blades. The containment case serves to channel incoming air through the fan so as to ensure that the bulk of the air entering the engine will be compressed by the fan. However, a small portion of the air is able to bypass the fan blades through a radial gap present between the fan blade tips and the containment case. Because the air compressed by the fan blades is used to generate thrust and feed the turbine section of the engine, engine efficiency can be increased by limiting the amount of air which is able to bypass the fan blades through this gap. Accordingly, the fan and containment case are manufactured to close tolerances in order to minimize the gap. However, manufacturing tolerances, differing rates of thermal expansion and dynamic effects limit the extent to which this gap can be reduced. Furthermore, during the normal operation of an aircraft turbofan engine, the fan blades may rub the containment case as a result of a hard landing or a hard maneuver of the aircraft. Any rubbing contact between the fan blade tips and the containment case will abrade the tips of the rotors, tending to further increase the gap between the containment case and blade tips, thereby reducing engine efficiency. The fan is circumscribed by a fan containment case such that the case is immediately adjacent the tips of the fan blades. The containment case serves to channel incoming air through the fan so as to ensure that the bulk of the air entering the engine will be compressed by the fan. However, a small portion of the air is able to bypass the fan blades through a radial gap present between the fan blade tips and the containment case. Because the air compressed by the fan blades is used to generate thrust and feed the turbine section of the engine, engine efficiency can be increased by limiting the amount of air which is able to bypass the fan blades through this gap. Accordingly, the fan and containment case are manufactured to close tolerances in order to minimize the gap. However, manufacturing tolerances, differing rates of thermal expansion and dynamic effects limit the extent to which this gap can be reduced. Furthermore, during the normal operation of an aircraft turbofan engine, the fan blades may rub the containment case as a result of a hard landing or a hard maneuver of the aircraft. Any rubbing contact between the fan blade tips and the containment case will abrade the tips of the rotors, tending to further increase the gap between the containment case and blade tips, thereby reducing engine efficiency.
In view of the above, it is well known in the art to cover the portion of the containment case adjacent the blade tips with an abradable material, such that the abradable material will sacrificially abrade away when rubbed by the fan blades. Inherently, as the abradable material is removed, the gap between the blade tips and the surface of the abradable material will increase, necessitating removal and replacement of the abradable material to maintain desirable aerodynamic efficiencies associated with a smooth abradable surface and a small gap between the abradable surface and the fan blade tips. Restoration of the abradable material also becomes necessary if damage has occurred from impacts with foreign objects.
A common technique for removing the abradable material is performed with handheld tools, such as an air chisel, after which sandpaper is used to achieve a smooth surface finish. While suitable for use on steel fan cases, air chisels are too aggressive for use on engines with aluminum cases. Aluminum fan cases must be removed from the fan frame, stripped of gearboxes, wire harnesses, controls, etc., and then centered on a turning machine to remove the old abradable material. Any damage that may occur to the base metal must be repaired before applying and bonding the new abradable material. The removal process can be time consuming and expensive, and requires a large maintenance facility to which at least the front of the engine must be transported for disassembly. Due to the special equipment required to perform the machining operation, a limited number of facilities are available for removing fan case abradable material. As a result, additional costs, scheduling and transport problems are common.
Accordingly, it would be desirable if an improved technique were available by which the abradable material of a fan containment case could be removed without requiring removal of the fan case from the engine, such that restoration can be performed in the field.
The present invention provides an apparatus and method for removing an abradable material from an interior surface of a fan containment case, without requiring removal of the fan containment case from its engine, e.g., a high-bypass gas turbine engine. The apparatus of this invention generally includes a frame, and means rotatably mounted to the frame for spraying an erosion media in a substantially radial direction relative to a horizontal axis of rotation of the spraying means. The erosion media impacts the abradable material on the interior surface of the fan containment case while the spraying means is positioned within the fan containment case adjacent the abradable material. The apparatus further includes means rotatably mounted to the frame with the spraying means for retrieving the erosion media after impacting the abradable material. Also provided is means mounted to the frame for centering the horizontal axis of rotation of the spraying means relative to the interior surface of the fan containment case, means for adjusting together the elevation of a horizontal central axis of the centering means and the horizontal axis of rotation of the spraying means relative to the frame, and means for positioning the spraying means adjacent the interior surface of the fan containment case.
A method made possible with the apparatus of this invention generally comprises the steps of adjusting the elevation of the spraying means and centering the horizontal axis of rotation thereof with the fan containment case to position the spraying means adjacent the abradable material on the interior surface of the fan containment case. The erosion media is then sprayed with the spraying means in a substantially radial direction relative to the horizontal axis of rotation of the spraying means, such that the erosion media impacts and erodes the abradable material on the interior surface of the fan containment case. In addition, the spraying means is rotated about the horizontal axis of rotation thereof so that the erosion media is sprayed at the abradable material along the entire circumference of the fan containment case. At the same time, the erosion media is retrieved after being sprayed and impacted against the abradable material.
In view of the above, it can be seen that a significant advantage of this invention is that the apparatus is capable of removing the abradable material from a fan containment case without requiring removal of the case from an engine. Instead, only the fan blades need be removed to gain access to the interior surface of the case. Another advantage of the invention is that the apparatus provides a means by which the spraying means is self-centered with respect to the interior surface of the fan containment case, and the operation of the spraying means can be controlled to follow a specific path along the inner diameter of the fan containment case. As such, the apparatus requires minimal setup, and can be operated in an automated mode without further human supervision. In a preferred embodiment, the apparatus is capable of removing substantially all of the abradable material in roughly a single rotation of the spraying means about its horizontal axis of rotation. With the use of appropriate abradable material and spray pressures, this operation can be completed in about one hour without damaging the substrate beneath the abradable material.
Other objects and advantages of this invention will be better appreciated from the following detailed description.